A study dealing with the ground state and excited state of behavior of a series of unsaturated 2H-arizines was initiated. The photobehavior of these systems was readily accounted for by carbon-carbon fission of the azirine ring to form nitrile ylides. The nitrile ylides were found to undergo intramolecular dipolar cycloaddition with neighboring double bonds to give a variety of heterocyclic rings. Results in our laboratory have shown that there are two pathways by which nitrile ylides react with multiple pi-bonds. The most frequently encountered path involves a parallel-plane approach of addends and can be considered to be an orbital symmetry allowed (4 plus 2)- concerted process. The other path, designated a 1,1-cycloaddition, operates only in certain intramolecular cases. It occurs when the p-orbitals of the olefinic group have been deliberately constrained to attack perpendicular to the nitrile ylide plane. A series of 2-allyl substituted 2H-azirines were studied and found to undergo smooth thermal rearrangement to afford 3-azabicyclo (3.1.0)hex-2-enes. The reactions can best be rationalized in terms of an equilibration of the 2H-azirine ring with a transient vinyl nitrene which subsequently adds to the adjacent pi-bond.